Smallest known exoplanet may actually be Earth-mass

The smallest planet around a normal star other than the Sun may be even smaller than first thought. A new analysis suggests the rocky body weighs just 1.4 Earths - less than half the original estimate. Observations over the next few months should test the prediction.

Most known "exoplanets" are huge gas giants, hundreds of times Earth's mass, and were discovered by detecting the wobble they induce in their parent stars.

The planet was detected using a technique called microlensing, in which one star passes in front of another as seen from Earth. Light from the background star is gravitationally bent and magnified for a period of days to weeks during the event. But if the nearer star hosts a planet, the planet's gravity can give an added boost to the background star's light for a few hours.

Heavier star, lighter planet

Analysing these events takes time, because there are so many variables to take into account, including the sizes of planet and star, their separation, and the distance from Earth.

Initially, the team believed that this host star was a brown dwarf - an object too puny to sustain nuclear fusion, as normal stars do. That suggested MOA-2007-BLG-192-L b weighed 3.3 Earths.

But more recent observations suggest the parent star is actually heavier than first thought - a type of star called a red dwarf, team member Jean-Philippe Beaulieu of the Paris Astrophysical Institute reported last week at a meeting of the Royal Astronomical Society in London.

That suggests the planet weighs just 1.4 Earths. In size terms, that makes it a near twin of our own planet, closer in mass than any known planet except Venus.

'Biggest goals'

Scott Gaudi of Ohio State University in Columbus, who is not on the team, says the new measurements "give a much more robust estimate" of the mass of the planet and its host star.

"The result is important because this is the lowest-mass planet yet detected, and is extremely close to the mass of the Earth," he says. "Obviously, finding a true Earth-mass planet is one of the biggest goals of searches for exoplanets. We are very close to that goal now."

The team plans to get more data on the parent star in April or May using the Very Large Telescope in northern Chile.

If their analysis is confirmed, it is an unclear whether the tiny planet could host any life. Because its host is a very dim red dwarf, the planet is likely to be frozen - even though it orbits at about the same distance as Venus from our Sun.

However, if the planet boasts a thick, insulating hydrogen atmosphere, it could sustain a habitable surface temperature that might be able to support some life of some kind.

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A tiny planet found last year might be even smaller than first thought, weighing just 1.4 times as much as Earth. It is thought to orbit a dim red dwarf star (Illustration: ESO)